1. Field of the Invention
The present invention relates to an imaging method and an imaging apparatus that can provide a small camera module having a high resolution.
2. Description of Related Art
Conventionally, in a solid-state imaging device such as a CCD or a CMOS used for a camera, an attempt has been made to increase the number of pixels with a view to achieving a higher image resolution. If the number of pixels simply are increased while maintaining the size of each pixel, the area of the solid-state imaging device expands, so that the number of devices obtained from a single wafer decreases and the yield lowers, resulting in a cost increase. Accordingly, the pixel size and the pixel pitch are reduced, thereby increasing the number of pixels. However, since the sensitivity and the saturation power generally are proportional to the pixel size, simply reducing the pixel size cannot break the limit of resolution improvement.
As a method for raising the resolution without increasing the number of pixels, a method called “pixel shifting” has been suggested. In this method, a subject image is formed on a solid-state imaging device while being shifted time-wise by ½ of a pixel pitch, thereby sampling the subject image at a spatial frequency higher than that determined by the pixel pitch. When the subject image is shifted in one direction, an image equivalent to that captured with twice as many pixels can be obtained apparently. Also, when it is shifted in two directions perpendicular to each other, an image equivalent to that captured with four times as many pixels apparently can be obtained.
Specifically, suggested methods for shifting a subject image to be formed on a solid-state imaging device relatively to the solid-state imaging device include a method of swinging the solid-state imaging device itself two-dimensionally using an actuator and a method of mechanically moving a specific optical component constituting an optical system. Other than these methods, a particularly promising method described in JP 2(1990)-52580 A or JP 4(1992)-63074 A is known. The method includes arranging a polarizer, a liquid crystal panel and a quartz-crystal plate in an optical system, in which the liquid crystal panel is electrically driven, thus shifting a subject image by a predetermined distance. FIG. 11 is a drawing for describing this principle.
FIG. 11 shows a quartz-crystal plate 113, which is ground obliquely with respect to its specific optical axis A 113 and transmits an incident light beam while shifting its traveling path according to the beam's polarization direction. In FIG. 11, in the light beam that has entered the quartz-crystal plate 113, a light component whose polarization direction is perpendicular to the surface of a sheet travels straight as an ordinary light beam, whereas a light component whose polarization direction is parallel with the surface of the sheet travels obliquely as an extraordinary light beam. Here, the extraordinary light beam is defined as a light beam that does not follow Snell's law. This makes it possible to shift the path of an incident light beam according to its polarization direction. The shifted amount D can be determined by the thickness of the quartz-crystal plate 113, and in this case, is set to be ½ of the pixel pitch of the solid-state imaging device (not shown) or an odd multiple thereof.
The following description is directed to the case where, in a light beam L from the subject side, only a light component whose polarization direction is perpendicular to the surface of the sheet is transmitted using a polarizer 111. The light beam that has passed through the polarizer 111 reaches a liquid crystal panel 112. The liquid crystal panel 112 is a twisted nematic liquid crystal panel. The polarization direction of the incident light beam is rotated by 90° by a liquid crystal sealed in the liquid crystal panel 112 when a drive voltage to be applied to the liquid crystal panel 112 is OFF, whereas the polarization direction does not change when the drive voltage is ON. Thus, the light beam that has entered the quartz-crystal plate 113 is shifted as an extraordinary light beam as indicated by a broken line when the drive voltage is OFF and travels straight as an ordinary light beam as indicated by a solid line when the drive voltage is ON.
Using such an optical system, the thickness of the quartz-crystal plate 113 is adjusted, thereby shifting the subject image that the light beam passing through the quartz-crystal plate 113 forms on the solid-state imaging device (not shown) by ½ of the pixel pitch (or an odd multiple of ½ of the pixel pitch) according to ON/OFF of the drive voltage of the liquid crystal panel 112. By turning the drive voltage ON/OFF at a high speed, reading electric signals from the solid-state imaging device in synchronization therewith and combining pieces of image information obtained from the ON state and the OFF state, respectively, it becomes possible to improve the image resolution up to a level equivalent to that obtained with a solid-state imaging device having twice as many pixels and the same size. The resolution is raised in this manner, so that a high-resolution image can be obtained easily even when a part of the obtained image is cut out and enlarged (zoomed).
In the pixel shifting according to the above-described method, by simply controlling ON/OFF states electrically, the subject image can be shifted very stably and at a high speed without using a mechanical driving part such as an actuator or a complex mechanism. JP 4(1992)-63074 A discloses a method of performing the pixel shifting two-dimensionally using this principle.
Further, JP 61(1986)-247168 A also discloses a similar method. In this document, although no optical element is clearly described, a polarizer is essential because a linearly polarized light beam is made to enter.
However, since all the methods disclosed in the above-mentioned three documents use a linearly polarized light beam, light components other than the linearly polarized light component in the light beam from the subject are lost. As a result, the subject image formed on the solid-state imaging device becomes dark, so that a S/N ratio lowers, resulting in an image with notable noise. Particularly when shooting a subject in a low light scene, the degradation of image quality becomes conspicuous.